In recent years, membrane bioreactors have become popular for treating wastewater. Membrane bioreactors combine biological treatment processes with membrane filtration to generally provide an advanced level of organic and suspended solids removal. These systems typically provide an advanced level of nutrient removal. Such membranes typically have pore sizes ranging from about 0.035 microns to 0.4 microns. This level of filtration provides high quality effluent to be transported through the membranes and generally eliminates the sedimentation and filtration processes typically used for wastewater treatment. Because the need for sedimentation is eliminated, the biological process can operate at much higher mixed liquor suspended solids concentrations. This can reduce the size of tanks required to carry out wastewater treatment.
One type of system includes at least one biological reactor and a membrane filtration tank disposed downstream from the reactor. A membrane module or cassette is typically submerged in the filtration tank. Mixed liquor is transferred from the reactor to the downstream filtration tank. The membrane module or cassette typically includes an array of submerged individual membrane filters. Mixed liquor is induced into the open space between the individual membrane filters, resulting in the mixed liquor being filtered and producing a permeate. The permeate is pumped or is flowing by gravity from the individual membrane filters and the filtration tank.
Typically the filtration tank is relatively large compared to the size of the membrane modules or cassettes. This means that when the membrane module or cassette is placed in the filtration tank, it is surrounded by mixed liquor or non-permeated mixed liquor. The term “non-permeated mixed liquor” means mixed liquor in the filtration tank that has passed through the membrane module or modules in the filtration tank. Practically, the non-permeated mixed liquor in the filtration tank tends to be recirculated multiple times through the membrane module or cassette. That is, the mixed liquor or non-permeated mixed liquor tends to move upwardly through the membrane module and exits the top of the module and then returns downwardly outside of the module, and then is induced back upwardly through the membrane module.
Typically, an air diffuser is disposed below the membrane module or cassette. The air diffuser tends to accelerate the non-permeated mixed liquor upwardly through the membrane module. The air bubbles created by the air being diffused gives rise to an air-lift effect within the membrane module or cassette. The air bubbles, after exiting the membrane module, tend to move upward in the direction of the water surface in the filtration tank while the non-permeated mixed liquor tends to turn and move downwardly in the opposite direction. The non-permeated mixed liquor is now flowing mainly outside of the membrane module towards the bottom of the filtration tank. In some cases, the non-permeated mixed liquor exiting the top of the membrane module has a velocity that is relatively high. When the velocity of the water exiting the top of the module is relatively high, the non-permeated mixed liquor tends to retain the air bubble and does not release the air bubbles to move to the top of the water surface in the filtration tank. The entrapped air bubbles reduce the velocity of the down flowing non-permeated mixed liquor, and by doing so the volume of the mixed liquor flowing upwardly through the membrane module is limited. This reduces the turbulence of the mixed liquor passing through the membrane module and tends to reduce finally the efficiency of filtration. Typically a portion of the non-permeated mixed liquor is coming from the treatment tank and another portion is flowing back into the treatment tank. This limits the increase of the mixed liquor suspended solids in the filtration tank.
Moreover, air diffused through the membrane modules functions to scour or clean the membrane filters within the respective modules. One of the challenges in designing a membrane bioreactor is providing a design where the air and mixed liquor that moves vertically through the filtration tank is generally uniformly distributed across the cross-sectional area of the membrane modules. Experience indicates that in many membrane bioreactors that air through the membrane modules is not uniform and that this impacts the cleaning or scouring efficiency of the air. When the air does not efficiently clean the membrane filters, this leads to increased down time for backwashing and cleaning the membrane filters.